1. Field of the Invention
The present invention relates to a process for producing a friction material for use as a disc pad, a brake shoe, or the like in the brakes of various vehicles, industrial machines, etc.
2. Related Art
Friction materials such as disc pads, brake shoes, and the like for use in the brakes of various vehicles, industrial machines, etc. have conventionally been produced by bonding two members to each other.
As shown in FIG. 10, a friction material is typically produced in the following manner. A metallic backing plate is produced which has been formed into a given shape by, e.g., sheet metal pressing and has undergone degreasing, priming, and coating with an adhesive (line (A)). Separately, a molding compound is prepared by mixing a fibrous reinforcement, e.g., heat-resistant organic fibers, inorganic fibers, or metallic fibers, with powdery starting materials comprising an inorganic/organic filler, a friction modifier, a thermosetting resin binder, etc., and sufficiently homogenizing the mixture by stirring, and then forming the molding compound at a room temperature and a given pressure to produce a preform (line (B)). The preform is heat-formed together with the metallic backing plate in a heat-forming step at a given temperature and pressure to integrally bond the two members to each other, and the integrated members are subjected to aftercuring and then final finishing to produce a friction material (line (C)).
In these conventional processes for producing friction materials, preforms and backing plates are produced in separate lines, as described above, and stored. Since the preforms and backing plates are generally stored at ambient temperature in factories or facilities, the temperature of the two kinds of members normally fluctuates with the ambient temperature of the storage place. Typically, the temperature of the stored members may fluctuate in the range of from 10 to 35.degree. C. over a year.
In the heat-forming step, a preform and a backing plate are fitted to a heat-forming mold and maintained at a given temperature for a given period while applying a pressure thereto.
Under these circumstances, the backing plate and the preform may in summer have a temperature of around 35.degree. C., which is the storage temperature when these members are fitted to a mold. On the other hand, these members can have a temperature of around 10.degree. C. in winter. The heat energy necessary for heat-forming hence varies considerably from season to season. If the same heat-forming conditions for summer are used in winter, the preform and the backing plate are underheated, often resulting in separation between the two members. Conversely, if the same heat-forming conditions for winter are used in summer, the preform and the backing plate are excessively heated, so that the preform part tends to develop defects such as swelling, cracks, etc.
When friction materials are produced under heat-forming conditions that are changed seasonally, troublesome temperature control is necessary.